The present invention relates generally to a spin coater track system, and more particularly, to a method and apparatus for cleaning a coating cup of a spin coater for preventing the deposition and accumulation of solidified coating material on an inside surface of the coating cup.
In manufacturing integrated circuits, a lithography process is frequently used for reproducing circuits and structures on a semiconductor substrate. As a first step in a lithography process, a photoresist layer is first coated onto a semiconductor substrate such that an image can be projected and developed thereon. The photoresist material is a liquid that is coated in a very thin layer on top of the substrate. In a conventional process for applying a photoresist coating material to a substrate, a spin coating apparatus is normally used. The spin coating apparatus is a sealed chamber constructed by an upper compartment, a lower compartment and a circular-shaped, rotating platform that has a diameter slightly smaller than the diameter of a semiconductor substrate. The rotating platform is a vacuum chuck since vacuum is applied to the platform for holding the semiconductor substrate securely during a spin coating process. The rotating platform is positioned in the coating machine such that a semiconductor substrate may be placed on top horizontally. During the coating process, the bottom or uncoated surface of a substrate contacts the rotating platform. A suitable vacuum is then applied to the bottom surface of the substrate such that it stays securely on the vacuum chuck even at high rotational speeds. The rotating motion of the vacuum chuck is achieved by a shaft, which is connected to the vacuum chuck and powered by a motor.
In a typical photoresist coating process, a desirable amount of a liquid photoresist material is first applied to a top surface of the substrate from a liquid dispenser that is mounted on a track while the substrate is rotated at a low speed on the vacuum chuck. The photoresist liquid spread radially outward from the center of the semiconductor substrate where it is applied towards the edge of the substrate until the entire top surface of the substrate is covered with a thin layer. Excess photoresist liquid is spun off the rotating wafer during the photoresist coating process. The rotational speed of the vacuum chuck and the amount of the photoresist liquid applied at the center of the substrate can be determined and adjusted prior to and during an application process such that a predetermined, desirable thickness of the photoresist is obtained. The rotational speed of the vacuum chuck is normally increased at the end of the application process to ensure that the entire surface of the substrate is evenly coated with the photoresist material.
A typical process flow chart illustrating a spin coating process 10 is shown in FIG. 1. In this spin coating process, a liquid material is first dispensed in step 12 by depositing a predetermined amount of liquid at or near the center of the wafer. The amount of the liquid can be suitably controlled by adjusting the flow rate through a dispensing nozzle from which the liquid is dispensed. The flow rate can, in turn, be controlled by a pressure existing in a liquid reservoir tank.
The wafer turns on a wafer pedestal at a rotational speed between 2000 and 3000 rpm when the liquid material is dispensed at the center of the wafer. The liquid material is then spun-out in step 14 by centrifugal forces from the center toward the edge of the wafer uniformly over the entire wafer surface. After all the liquid material is spun-out and the edge of the wafer is fully covered, the solvent contained in liquid has at least partially vaporized and form a solid coating on the wafer surface. After the spin-out step 14 is completed, an edge bead rinse process of step 16 is carried out at the edge of the wafer surface, i.e. an area of approximately 2-3 mm from the edge of the wafer, to wash away material deposited at such area. At this stage of the process, the material has mostly solidified and thus the edge bead rinse process is not always effective. After the edge bead rinse step 16, the backside of the wafer is rinsed by a different jet of cleaning solvent to wash away material deposited at the wafer backside and the interior surface of the coating cup. This is shown as step 18 in FIG. 1. However, following the backside rinse procedure, some photoresist material may still remain on the interior surface of the cup. The wafer is then spun-dry in step 20 to complete the coating process.
A typical spin coating apparatus 22 for coating photoresist on a semiconductor substrate is shown in FIG. 2. The spin coating apparatus 22 consists of a coater cup 28 and a rotating platform 30, i.e. a vacuum chuck, positioned at the center of the coater cup 28 for supporting a semiconductor wafer 26 on a top surface 24 of the vacuum chuck 30. The vacuum chuck can be rotated by a shaft 32, which is connected to an electric motor (not shown). The coater cup 28 is provided with a spent photoresist drain pipe 34. The spent photoresist drain pipe 34 is used to drain away photoresist liquid 36 that spun off the substrate during a coating operation.
In the operation of the conventional spin coating apparatus 22 of FIG. 2, the rotating platform 30 is first loaded with a semiconductor wafer 26 on top. A liquid dispenser 18 then approaches the center of the wafer 26 and applies a predetermined amount of a liquid photoresist material to the center of the substrate. The vacuum chuck 30 then spins to spread out the photoresist material to evenly cover the top surface of the wafer 26. Extra photoresist liquid 36 is thrown off the substrate surface and drained away by the drain pipe 34.
In the conventional spin-coating process of FIG. 1 utilizing the apparatus of FIG. 2, the process results in a significant waste of the liquid coating material since a large amount of the liquid coating material is thrown off the wafer surface during wafer spinning. The liquid coating material thrown off the wafer surface adheres to an interior surface of the cup sidewall and thus forming a donut-shaped ring of solidified photoresist material on the interior surface of the sidewall. The solidified photoresist material becomes a major source of particle contamination during subsequent coating processes for the photoresist. Moreover, the solid particles of the photoresist material may also cause a blockage of a drain pipe that is normally located at the bottom wall of the cup for draining away the liquid coating material. It is a difficult task for un-blocking or cleaning the drain pipe to ensure a free flow of the spent photoresist material.
Accordingly, there is a need for a spin coating apparatus for coating a liquid material on a wafer that does not have the drawbacks or shortcomings of the conventional spin-coaters.